Dielectric insulation media in liquid or gaseous state are conventionally applied for the insulation of an electrical active part in a wide variety of electrical apparatuses, such as switchgears or transformers.
In medium or high voltage metal-encapsulated switchgears, for example, the electrically active part is arranged in a gas-tight housing, which defines an insulating space, said insulation space comprising an insulation gas usually with a pressure of up to several bars and separating the housing from the electrically active part, thus preventing flow of electrical current between housing and active parts. Metal-encapsulated switchgears allow for a much more space-saving construction than switchgears which are mounted outdoors and are insulated by ambient air. For interrupting the current in a high voltage switchgear or for extinguishing fault arcs, the insulating gas further functions as an arc extinction gas.
Sulphur hexafluoride (SF6) is a widely used dielectric insulation gas with excellent dielectric strength, when used for insulation purposes, and also with excellent arc extinction strength, when used for example in a circuit breaker for extinguishing of switching arcs or for example in a gas-insulated substation (GIS) for extinguishing fault arcs. Tetrafluoro methane is particularly useful for switching application because of good arc interrupting properties and very low boiling point of −128° C. at 1 bar. The dielectric strength or pressure-reduced breakdown field strengths are about 84 kV/(cm*bar) for SF6 and about 31 kV/(cm*bar) for CF4.
For example, WO 2008/073790 discloses a dielectric gaseous compound which—among other characteristics—has a low boiling point in the range between −20° C. to −273° C., is preferably non-ozone depleting and which has a GWP of less than about 22,200 on a 100 year time scale. Specifically, WO 2008/073790 discloses a number of different compounds which do not fall within a generic chemical definition.
Further, U.S. Pat. No. 4,175,048 relates to a gaseous insulator comprising a compound selected from the group of perfluorocyclohexene and hexafluoroazomethane, and EP-A-0670294 discloses the use of perfluoropropane as a dielectric gas.
EP-A-1933432 refers to trifluoroiodomethane (CF3I) and its use as an insulating gas in a gas-insulated switchgear.
In the search for a suitable substitute of conventional insulation gases, such as SF6 or air, it has been found that by using fluoroketones having from 4 to 12 carbon atoms, an insulation medium can be obtained which has high insulation capabilities, in particular a high dielectric strength, and at the same time an extremely low global warming potential. This invention has previously been filed as international patent application No. PCT/EP2009/057294.
German Utility Model DE 10 2009 009 305 and German Patent DE 20 2009 025 204 B3 also relate to a switching device having an encapsulation that is filled with a filling medium comprising or consisting of a fluoroketone.
Despite of the good dielectric strength of the fluoroketones according to international patent application No. PCT/EP2009/057294, the insulation performance of the respective insulation medium comprising the fluoroketone is often limited due to the relatively high boiling points of the fluoroketones.
This is particularly the case for applications in a low temperature environment. In this case, only a relatively low saturated vapour pressure of the fluoroketone can be maintained without fluoroketone becoming liquefied. This limits the achievable fluoroketone molar ratio in the gaseous phase and would make necessary an increased filling pressure with conventional insulating gases.
For example, the minimal permissible operating temperature of a high or medium voltage gas-insulated switchgear (HV-GIS or MV-GIS) can be typically −5° C. At this temperature, for obtaining a dielectric performance comparable to conventional high-performance insulation media, the required filling pressure of an insulation medium comprising e.g. a fluoroketone having 6 carbon atoms, e.g. C2F5C(O)CF(CF3)2 or dodecafluoro-2-methylpentan-3-one, may still be relatively high and could exceed the filling pressure that can be withstood by usual housing constructions, which is typically about 7 bar for HV GIS applications.
Alternatively or additionally to increasing the filling pressure, the system can be heated (as shown in our PCT/EP2009/057294). If using for example a pure fluoroketone having 6 carbon atoms, e.g. C2F5C(O)CF(CF3)2 or dodecafluoro-2-methylpentan-3-one, as the insulation medium, heating to more than 50° C. would be required to achieve a sufficient saturated vapour pressure of the fluoroketone and to obtain the desired insulation performance for more demanding high voltage applications. Such heating is not always feasible or recommended both for economic and ecologic reasons.
In the article by Yamamoto et al, “Applying a Gas Mixture Containing c-C4F8 as an Insulation Medium”, IEEE Transactions on Dielectrics and electrical Insulation, Vol. 8 No. 6, December 2001, it is disclosed that mixtures of c-C4F8 together with nitrogen, air or carbon dioxide show a nonlinear increase in dielectric strength over the concentration-weighted arithmetic sum of dielectric strengths of the components.